Problems associated with currently used techniques for packaging semiconductor devices include:
1. Cracked die--especially at the corners. The problem is due to stress on the die in the package. If the die is in a ceramic package, the die cracks because of voids in the die attach or thermal expansion of the mount which is different from the die. If the die is in a plastic package the problem is due to the plastic stress on the die. PA1 2. Shifted metal--the metal on the device is cracked, broken or distorted by the stress of the plastic package against the die. In a worst case scenario, the metal is sheared off of the die. PA1 3. Sheared ball bonds--the plastic package sheers the bond off of the die because of the plastic stress on the ball bond. PA1 4. Filler induced damage--plastic packaged devices can be damaged by filler in the plastic. The filler presses against the die, breaking the protective overcoat of the die. The die then fails due to contamination. PA1 5. The bond wire touches the die edge--the bond wire touching the die edge causes the die to short to the wire. The die then becomes nonfunctional. In a less extreme case, the wire is just close to the edge resulting in a long term reliability problem. Reasons for the bond wire touching the die range from miss bonding to wires caught in the "PIX" coating during cure. PA1 6. Die edge touches the attach substrate--on flip chip technology, one of the problems that must be resolved is the height of the bump used to attach the die to the substrate. If the bump is too high, multiple bumps can not be put close enough together to be of use. If the bumps are too low, the die edge touches the substrate. PA1 7. Lack of space for bond pads--there is no longer enough room for the bond pads around the outside edge of the die. Current bonding techniques require that no active circuitry be under the bonding pad. PA1 8. Noise on the ground and power lines--the inductance of the power lines limits the current to the device in peak power surges. The device needs more and better power/ground lines. PA1 9. Device noise--capacitors are needed on the device to reduce the noise in the device. PA1 10. Broken bond wire--during centrifuge tests, the bond wires can break.
Various techniques have been utilized to to resolve some of these problems. To prevent the filler induced damage, different filler sizes and shapes have been tested. The introduction of PIX to improve the alpha sensitivity of the DRAM's has helped reduce filler induced damage, but not enough.
To stop stress on the die, different plastic compositions have been tried. The most common effort has been to match the coefficient of expansion with that of the die. Two techniques come close to resolving this particular problem. The first technique requires a heat spreader below the die. This technique, however, does not provide protection for the bonding wires. In the second technique, a ceramic cap is placed over the die to prevent the plastic from coming in contact with the die on the top side. The cap is needed to enclose a tall arch for the bonding wires attached from the bonding pad to the lead frame. The resulting cap, however, is costly and does not help with heat spreading.
For very large die sizes having many bond pads, there is no more room for additional bond pads on the die edge. For some devices, this results in bond wires forming high loops and the wire traverses of long distances over the die. The long bond wire over the die can be moved by the plastic or by other mechanical means. The wire can be forced to the edge of the die by the mechanical movement and cause a short because the die edge is bare, broken silicon left after die scribe and break.
It is the principal object of this invention to provide an improved method of plastic packaging using heat spreaders and CVD deposition of wire bonds.